Multiple lines of evidence support the viral and the immunologic hypotheses in Multiple Sclerosis (MS). However, no one virus has been reproducibly isolated from MS patients, and the specificity of antibody produced within the CNS in MS is undefined. Although soluble immune complexes (ICx) have been detected in low concentration in CSF of patients with MS, the antigen moiety in these complexes is unknown. The antigen component must be of either exogenous or endogenous origin. Characterization ICx components might implicate exogenous factors (e.g. virus) or endogenous antigens in pathogenesis of MS. Quanlities of ICx available in CSF from MS patients are insufficient for traditional biochemical approaches to antigen characterization, and immunochemical approaches require a prior notion as to the nature of the antigen present. To circumvent these problems, we have developed an approach to ICx characterization that is both sensitive and direct. We propose to apply this method to the study of ICx in CSF of MS patients. We have shown that anti-IgG conjugated to ferritin (anti-IgG-F) can be used to separate soluble ICx from solution, even in the presence of a 200-fold excess of monomeric IgG. Further, we have utilized anti-IgG-F to isolate model ICx on a sucrose density gradient and to simultaneously label model ICx with ferritin, a distinctive EM marker. We have used negative stain EM preparative techniques for high resolution ultrastructural study of the immunologically labeled, isolated model ICx. This simple and direct technique will be applied to the ICx of patients with MS and other neurologic diseases.